One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between downhole locations themselves. For example, in the oil and gas industry it is desirable to communicate data generated downhole to the surface during operations such as drilling, perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to downhole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable downhole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded analog or digital signals.
In oilfield exploration and production operations, it is a common industry practice to perform downhole testing that provides information relevant to the borehole (e.g., downhole temperature, pressure, fluid flow, viscosity, etc.). This testing may be performed by deploying tools and/or a bottom hole assembly downhole, in which information and data from the tools and assembly may be recovered later after the tools have been retrieved back at the surface. However, with this testing method, if the information and data recorded by the tools and bottom hole assembly are corrupted and/or insufficient, such as by having a failure within the testing equipment, this insufficiency within the data may not be apparent until after the tools have been retrieved back at the surface. Further, while the downhole tools are being operated, an oil-rig operator may not have access to the information being recorded downhole until the retrieval of the downhole tools at the surface. As such, the operator may not be able to compensate and adjust the downhole conditions within the borehole until after the tools and/or assembly has been retrieved.
Other testing methods have also been developed to provide two-way communication between the borehole tools and/or bottom hole assembly and the surface. One method involves placing a cable into the borehole that runs from the surface near the drilling rig down to the data recording tools. However, such a use of a cable may obstruct the flow of fluids within tubulars downhole. Further, the cable would have to be safely and properly managed, as the cable could easily be damaged while either inside or outside of the tubulars. Furthermore, the cable may also obstruct the disconnection of the downhole tubulars from the surface in the case of an emergency disconnection between the two.
Other methods have then been developed to provide wireless two-way communication between the borehole and the surface, such as by using acoustic and/or electromagnetic signals to enable communication. For example, referring to FIG. 1A, a schematic view is shown of a downhole communication system 101. The communication system 101 includes a section having one or more downhole tools 103, such as an MWD tool recording and transmitting data. The recorded data from the downhole tools 103 may then be sent to other tools adjacent thereto, or the data may be sent to the surface for evaluation.
As mentioned, when using the downhole tools 103 to transmit data, the data may be transmitted wirelessly using acoustic and/or electromagnetic signals. The electromagnetic or acoustic wireless signals may be used for shorter ranged applications, such as transferring data within and between downhole tools 103 that are adjacent to each other, commonly referred to as the “short hop section.” Alternatively, or in addition thereto, the electromagnetic or acoustic signals may be used for longer ranged applications, such as transferring data between the downhole tools 103 and the surface, commonly referred to as the “long hop section.”
When the distance between the downhole tools 103 and the surface is too far to transmit the wireless signal via the short hop section, then the long hop section may be used to receive the data signals from the short hop section and re-transmit the signals at a higher level and/or higher power. These signals re-transmitted by the long hop section may then be received by the surface, thereby having the signals from the downhole tools 103 transmitted to the surface.
To re-transmit the signals from the short hop section, the long hop section may include one or more devices, commonly referred to as repeaters, disposed downhole that receive and re-transmit the wireless signals. For example, as shown in FIG. 1A, five repeaters 105 have been added to the communication system 101 to transmit and carry the data from the downhole tools 103 to the surface.
Furthermore, in another method, a wireless two-way communication system may include more than one short hop section, such as by having multiple tools disposed downhole in different sections within a borehole. In such a system, each of the different short hop sections may transmit information and data signals therefrom to adjacent short hop sections and/or adjacent long hop sections. For example, referring to FIG. 1B in another schematic view, multiple downhole tools 103 are disposed downhole at different sections such that the data from each of these tools 103 may be transmitted to the surface. As such, multiple repeaters 105, particularly six repeaters 105 in this embodiment, may be used to provide communication between the short hop sections and the long hop sections, thereby transmitting the data from each of the downhole tools 103 to the surface.
However, in such wireless communication systems, the failure of one or more of the components within the long hop section (e.g., repeaters within a long hop section) may result in a complete loss of communication within the system. For example, the system may no longer be able to re-transmit signals within the long hop section of the communication system. This may necessitate the redeployment of additional communication components downhole, thereby resulting in additional costs (particularly within a rig environment) and increasing the time until production from the well is received.